What is Soil Exploration or Soil Investigation in Construction?

The field and laboratory investigation performed on subsoil to obtain information on its strength and compressibility is called soil exploration or soil investigation. These tests determine several stress parameters of the soil that help in the analysis of the foundation and other earth-retaining structures. 

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Soil Borings for Testing
Soil Borings for Testing
Image Credits: Foundation Masters

Soil exploration is performed based on the magnitude and importance of the project. For example, projects like buildings, and power plants demand soil exploration within a localized area. Transmission lines, roads, and railway lines require soil exploration along a narrow path. Hence, the principle of soil exploration remains the same, but the program and methodologies vary from project to project. 

In this article, we will explore the objectives and various steps and methods followed in soil exploration for a construction project. 

Objectives of Soil Exploration

  • To identify key soil characteristics, such as compressibility, strength, and hydrological conditions, which have an impact on the design and safety of construction projects.
  • To select the appropriate type and depth of foundation for a specific structure.
  • To determine the quantity and quality of materials required for construction.
  • To establish the foundation's capacity to bear loads.
  • To gauge the depth of the groundwater and the potentially corrosive effects of water and soil on foundation materials.
  • To predict the expected settlement of a building. To investigate the causes of failures in existing structures.
  • To formulate construction strategies tailored to changing subsurface conditions. To assess the lateral ground pressure exerted on retaining walls and abutments.
Soil Testing 
Image Credits: Mikula Contracting

Stages in Soil Exploration Program

A planner before developing a soil exploration program investigates the type, size, and importance of the project. Small projects demand only preliminary investigation, while some complex ones require both preliminary and detailed investigation. Let's check the stages in site exploration:
  1. Site Reconnaissance
  2. Preliminary Investigation of Soil
  3. Detailed Investigation of Soil
  4. Report

1. Site Reconnaissance

Site reconnaissance is the initial step in investigating a location. During this phase, the site is visually inspected, and information about its natural features is gathered. The key things we look for during site reconnaissance include: 
  • Checking for drainage ditches and areas where waste is dumped.
  • Determining the groundwater level by examining wells on the site.
  • Identifying the presence of springs, swamps, and similar features.
  • Noting high-water marks on bridges and tall buildings to understand flood levels.
  • Assessing the types of plants and the quality of the soil in the area.
  • Reviewing historical data on events like landslides, floods, and soil cracks in that region.
  • Analyzing aerial photos, building blueprints, geological maps, and other documents related to the site.
  • Examining deep excavations to understand the layers of soil.
  • Observing cracks in existing structures that indicate settling.

This initial site assessment helps us gather crucial information for further investigation

2. Plan for Preliminary Investigation of Soil

During the preliminary site investigation, a few boreholes were drilled from distributed locations for sampling. The sample collected is taken for field and laboratory tests to determine the strength characteristics of the subsoil. These lead to preliminary drawings and design. If the study is unsuitable, the procedure is repeated at a different location. 

Soil Exploration in Construction
Soil Sample for Soil Investigation and Testing

3. Plan for Detailed Investigation of Soil

The planning for a detailed soil investigation includes the following steps:
  • A detailed study of the geographical condition of the area. This includes the Collection of Available information from the site like topography and geographical maps, Topographical features, hydraulic conditions, water table fluctuations, flood conditions, etc, and access to the site.
  • Preparing the layout plan of the project
  • Preparing borehole layout plan - depth and number of boreholes
  • Marking on layout, additional types of soil investigation.
  • Prepare specifications and guidelines for the field investigation of various elements of soil investigation.
  • Prepare specifications and guidelines for laboratory testing of the samples collected, presentation of the field and laboratory test results, writing of reports, etc.

4. Report of Site Exploration

Following the completion of either initial or comprehensive site exploration procedures, it is imperative to create a report. A sub-soil investigation or exploration report typically comprises the subsequent sections:
  1. Introduction: This part serves as an opening to the report, presenting its purpose and context.
  2. Site Investigation Scope: In this section, the report outlines the precise objectives and scope of the site investigation.
  3. Description of the Intended Structure: This segment offers insight into the proposed construction project, elucidating its purpose and the rationale behind the site investigation.
  4. Site Reconnaissance Findings: Here, you'll find observations and findings from the initial site assessment, including details about the site's natural characteristics.
  5. Site Exploration Particulars: This portion provides information on the site exploration process, detailing the quantity, location, and depth of boreholes, along with specifics regarding soil sampling procedures. 
    Bor Log Data

  6. Methods Employed in Site Exploration and Their Outcomes: This section elucidates the techniques applied during site exploration and summarizes the results they yielded.
  7. Laboratory Tests and Their Outcomes: The report elaborates on the laboratory tests conducted on soil samples obtained during exploration and presents the results, which are crucial for assessing soil properties.
  8. Groundwater Table Information and Location: This segment provides data on the depth and position of the groundwater table, essential for construction planning.
  9. Recommendations for Potential Improvements: If site conditions necessitate enhancements to facilitate construction, the report outlines recommended methods or strategies to address these requirements.
  10. Suggested Foundation Types and Structural Aspects: Based on the findings from the investigation, the report offers suggestions for the most suitable foundation types and imparts crucial structural details to consider in the construction project.
  11. Conclusion: The report concludes by summarizing the key findings and recommendations, emphasizing their significance.

This structured report offers a comprehensive overview of the site investigation process, its outcomes, and the guidance required for successful construction endeavors, with an emphasis on safety and structural integrity.

Methods Used in Site Exploration



1. Direct Methods of Soil Exploration

·       Test or Trial Pits

Simple excavations for shallow exploration; cost-effective.

2. Semi-Direct Methods of Soil Exploration

·       Auger Boring

Drilling holes above the water table in soft soils.

·       Auger and Shell Boring

Using casing for support in sand and clay; power-driven.

·       Wash Boring

Quick drilling with water pressure; samples for testing.

·       Percussion Boring

Repeated soil fragmentation with a drilling bit.

·       Rotary Boring

Deep boreholes in challenging formations using drilling mud.

o   Mud Rotary Boring

Drilling deep holes through various ground conditions.

o   Core Drilling

Collecting undisturbed soil samples, often in soft rock.

3. Indirect Methods of Soil Exploration

·       Sounding or Penetration Tests

Calibrated tips for estimating soil properties.

·       Geophysical Methods

Determine soil strata limits using seismic or resistivity.

Based on the proficiency of the planner in the soil exploration process, an intelligent, practical, and pragmatic plan can be prepared for the particular project.

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